At present, paint and other coatings are baked or cured in a radiant oven, wherein the walls and/or floor are heated by combustion burners. In a typical automotive application, the painted vehicle body is conveyed through the radiant oven and the opposed side walls and generally the floor of the oven are heated by combustion burners which direct hot air into rectangular enclosures defining the side and bottom walls of the oven. This method of heating requires flame sensors to avoid a potential explosion which automatically turn off the flow of combustible gas to the burners in the event that the burner fails and combustible gas is directed into the enclosures thus shutting down the oven. The enclosures must then be purged of combustible gas before restarting the burners and restarting the oven. As a safety feature, if any of the flame sensors fail, the burners are automatically shut down for maintenance.
As will be understood by those skilled in this art, restarting a paint oven after shut down because of a failure of either the burner or a flame sensor requires time and thus considerable expense, but safety precautions must be maintained to avoid a hazardous condition. Radiant tube heaters have previously been used in automotive paint ovens, but only for supplemental heating of the rocker panels. It has now been proposed by General Motors Corporation to utilize radiant tube heaters that are pulse-fired to provide uniform surface temperature on the radiant tubes and the burners may be mounted external to the oven burner enclosures, thus allowing the use of the radiant tube heaters throughout the entire length of the oven. This method of heating a paint oven has several important advantages over the prior art. First, the potential for explosion is substantially eliminated because the radiant tube heaters may be formed of a schedule 40 pipe having a diameter of about six inches and a wall thickness of 0.280 inches, which are not subject to explosion even if the burners fail. The timing of the pulse-fire may be controlled by the surface temperature of the radiant tubes and a continuous spark may be utilized to ignite the combustible gas and air mixture, wherein combustible gas and air are mixed with a small portion of excess air and the mixture is at or near stoichiometric conditions. Further, a conventional paint oven has two sections, including a bring-up section and a hold section, wherein the temperature of the item being cured is brought to the paint curing temperature in the bring-up section and the curing temperature is maintained in the hold section as the painted substrate is moved through the oven. In a conventional paint oven, each section or zone uses a gas-fired heater to heat the zone as described above.
Although the described radiant tube oven provides important advantages over the prior art, further improvements are required to fully implement this invention, including improved control of the temperature of the painted substrate, such as an automotive body, to avoid overheating of areas adjacent the radiant tubes and to provide uniform heating of the substrate and improve efficiency, including conservation of energy. These advantages are provided by the radiant tube heat oven of this invention as described below.